The present invention relates to an apparatus for the inspection of heat exchanger tubes in an area at a given distance from the end of said tubes, by means of a probe, such as an eddy current probe, which performs a helical movement.
Throughout the description, the term "heat exchangers" is used in a general sense to describe all types of exchangers having a bundle of tubes in which circulates a first fluid exchanging its heat with a second fluid circulating about the tubes. This term also covers steam generators.
Heat exchangers generally comprise either straight tubes, or U-shaped tubes. In both cases, the ends of the tubes are sealingly fixed to one or both tube plates, whose thickness is relatively great (e.g. 540 mm for the tube plate of a U-shaped tube steam generator equipping a nuclear power station). The tubes are fixed to the tube plates in such a way that the tube passes through the entire plate and its end is flush with the outer face of the plate defining the collector, (or water box in a steam generator). The actual fixing takes place on the one hand by welding or brazing at said end, and on the other hand by expansion over the entire thickness of the tube plate.
When the fluids circulating in the heat exchanger are corrosive fluids under the operating conditions, as is in particular the case for the heat exchangers used in nuclear power stations, the exchanger tubes form particularly fragile members, particularly at the expanded zone and the transition zone between the expanded parts and the unexpanded parts of the tubes. This is due on the one hand to the tensions produced in the tubes during expansion, and on the other to deposits of corrosive substances, which can occur level with the tube plate.
It is therefore particularly important to periodically check or inspect the tubes of heat exchangers, particularly in the vicinity of the inner face of the tube plate. It is also useful to check the tube plate during the same operation.
In general terms, it is known to check heat exchanger tubes by introducing into them an eddy current probe and by moving the latter forwards and backwards in order to detect defects or faults in the tube.
It is also known to check areas close to the welded or brazed joint by which the end of the tubes is fixed to the outer face of the tube plates, by using an eddy current probe, which carries out a helical movement, as described e.g. in the Intercontrole French Patent Application No. 8,011,295 of May 16, 1980.
In order to check and inspect the tubes at the point where they are fixed by expansion to the tube plate, it would appear necessary to use a device comparable to that known for inspecting the area close to the welded or brazed joint fixing the end of the tubes to the tube plate, i.e. a device comprising an eddy current probe, which performs a helical movement. However, the device described in French Patent Application No. 8,011,295 of May 16, 1980 cannot be used for inspecting the expanded area of the tubes. Thus, bearing in mind the significant thickness of the tube plate (approximately 540 mm for a steam generator equipping a nuclear power station), the probe would then have to be fixed to the end of a rigid rod having a length similar to the thickness of the plate. When the probe is removed from a tube in order to permit its movement up to another tube, it would then be necessary to have a free space, at least equal to the said length facing the front face of the tube plate. However, in existing heat exchangers, the collectors for water boxes are generally hemispherically shaped, in such a way that even if such a space is available in the central part of the tube plate, it is not present on its periphery. Thus, such a device would not make it possible to check tubes located on the periphery of the tube plate, which is obviously unsatisfactory.
Thus, there is a need for a new apparatus making it possible to check the expanded area of heat exchanger tubes. In addition, the helical movement of the probe must be very precise and the angular movements of the probe necessary for the quality of the inspection must be as regular as possible. Thus, the probe must be able to move in an extremely accurate and regular helical manner (pitch of approximately 1 mm over a distance of approximately 50 mm).
The solution of the latter problem is made particularly difficult in view of the impossibility of using a device having a rigid connection between the probe and the means controlling its helical displacement.